This invention relates to a device using single-component developing material for developing an electrostatic latent image recorded on a photoconductive layer.
In the art of xerography as discussed in U.S. Pat. No. 2,297,991 to Carlson, a xerographic plate, which comprises a layer of photoconducting and insulating material on a conducting backing, is given a uniform electric charge over its surface and is then exposed to the subject matter to be reproduced. This exposure results in discharge of the photoconductive plate whereby an electrostatic latent image is formed. The latent charge pattern is developed or made visible with a charged powder. Thereafter, the developed image is transferred to a support member to which it is fixed. Controlled development of electrostatic latent image can be accomplished by several techniques including cascade, magnetic-brush, liquid-dispersion development, etc. Another important development technique is called as "transfer development" which is, for example, disclosed in U.S. Pat. No. 2,895,847 to Mayo. This development process employs a support member such as a "donor" which carries a layer of toner particles to be brought into close contact with the electrostatic latent image to be developed.
It is to be noted that the term "transfer development" is generec to development techniques where (1) the toner layer is out of contact with the photoconductor and the toner particles must traverse an air gap to effect development, (2) the toner layer is brought into rolling contact with the photoconductor to effect development, and (3) the toner layer is brought into contact with the imaged photoconductor and skidded across the imaged surface to effect development. Transfer development has also come to be known as "touchdown development".
A serious problem which occurs with transfer type development is fog or background development. In order to minimize background development, there is proposed, in U.S. Pat. No. 2,289,400 to Moncrieff-Yeates, an out of contact transfer development system in which toner particles tranverse an air gap between the doner and the xerographic plate to develop the electrostatic latent image disposed on the xerographic plate. However, the special positioning of the doner and the xerographic plate in relation to each other is critical. For example, the length of the air gap or development gap must be adjusted at a value less than 0.05 mm and preferably less than 0.03 mm. This adjustment involves considerable difficulty in maintaining the xerographic plate and the doner within the required range of mechanical accuracy. Several attempts have been made to overcome the difficulty. For example, in U.S. Pat. Nos. 3,866,574 to Hardennrock, 3,890,929 to Walkup, and 3,893,418 to Liebman, a pulse generator source is employed for applying pulsed bias potentials to create electrical fields across the air gap between the toner carrier member and the latent image bearing member. Particularly, the Hardennrock patent discloses that optimum line development is effected with a minimum of background deposition when the three conditions are established, that is, when the air gap length (g) is in the range of 0.05 mm to 0.18 mm, the AC electric voltage frequency (f) is in the range of 1.5 kHz to 10 kHz, and the pulsed bias potential (V.sub.p-p) is less than 800 volts.
Furthermore, the conventional transfer type development systems as disclosed in the Hardennrock patent utilize the electrostatic forces of the latent image to overcome the carrier-toner bond and attract toner particles onto the image areas. The toner can transfer from the doner to the image areas on the xerographic plate across the air gap when the intensity of electrostatic forces associated with the latent image exceeds a threshold value which may be referred to as toner transfer threshold value. Although the toner bonding forces vary from one toner particle to another due to the dispersion of physical and chemical properties of the individual toner particles, they are distributed in a narrow range around a fixed value. Consequently, development is effected in such binary form fashion that toner particles are deposited on the image areas producing electrostatic forces exceeding the toner transfer threshold value, while no toner particle is deposited on the areas producing electrostatic forces less than the threshold value. In other words, the characteristic curve representing image density with respect to surface potential has such a great gradient (.gamma.) as to cause poor continuous-tone development. In addition, the characteristic curve has such a great gradient (.gamma.) as to allow only a part of toner particles to traverse the air gap if the amplitude of the pulsed bias potential (V.sub.p-p) is less than 800 volts even though the toner bonding forces are distributed in a wide range.
Japanese Patent Publication No. 58-32375 discloses a transfer type development method which improves the quality in continuous tone images by applying a low-frequency bias voltage to create alternative electric fields across the air gap between the toner carrier and the xerographic plate. The toner transfers from the toner carrier to the xerographic plate during one half cycle of applied voltage, this cycle being termed to toner transfer cycle. The toner transfers back to the toner carrier from the xerographic plate during the second cycle which is termed to toner counter-transfer cycle. The Japanese Publication describes that the quality of continuous tone images can be improved to a considerable extent by repetitive transfer and counter-transfer cycles when the applied bias voltage is at a frequency lower than 1 kHz, while the effect is diminished when the biase voltage frequency is higher than 2 kHz. It is considered that application of low-frequency bias voltage to create alternative electrical fields across the air gap is effective to deposite toner particles on image areas in conformity with the latent image pattern with high fidelity to its surface potentials in the case where the toner bonding forces are distributed in such a narrow range as to effect binary-form development. However, the development method disclosed in he Japanese Publication is disadvantageous in that (1) the forces produced by the electrical fields associated with the image and non-image areas are not different on the toner carrier and (2) dot or screen pattern images cannot be reproduced with high fidelity since toner particles do not transfer along the electrical force lines, resulting in low resolution.
Therefore, the present invention provides an improved developing device which can achieve an excellent reproduction of dot or screen pattern images without degrading the quality of reproduction of line and solid images.